During the current grant period, we developed new methods for quantification of MAO A and B in human peripheral organs and carried out the first studies demonstrating a significant reduction in MAO in peripheral organs of smokers. We also developed a new radiotracer for the brain nicotinic receptor (nAChR) and successfully translated it from basic radiotracer research into a scientific tool for human neuroscience to characterize this molecular target, which underlies the addictive, protective and toxic effects of tobacco smoke. In parallel, our work took a new direction with the development of [11C]methylreboxetine and other labeled reboxetine derivatives as the first promising lead compounds for PET imaging of the brain norepinephrine transporter (NET). The involvement of the NET in the pathophysiology and treatment of ADHD, substance abuse, and depression, and as an important molecular target in drug R&D, has long been recognized However, in contrast to the dopamine (DA) and serotonin (SER) systems for which there are a number of suitable PF[ radiotracers for humans, brain imaging of NET has been hampered for over 20 years due to the lack of suitable: adioligands. Building on our long-standing track record in radiotracer development and translation, and the urgent need to develop and apply new radiotracers to investigate stimulant abuse and ADHD, we now propose to develop a series of C- 11 labeled reboxetine derivatives and other promising ligands, and to evaluate them in baboons (Specific Aim 1). The ability to map NET in vivo will provide the first opportunity to track its role in vivo, including the role of NET in stimulant drugs. We will also measure the dose-dependent blocking effects and duration of action on NET by cocaine and methylphenidate (MP) in baboons (Specific Aim 2). We will estimate dosimetry in the baboon model and carry out a toxicology study for IND application (Specific Aim 3) prior to translation and evaluation in healthy human subjects (Specific Aim 4). We will also measure the occupancy of the NET by oral MP at therapeutic doses in healthy humans (Specific Aim 4). Results from these studies will provide relevant clinical information regarding the role of NET in ADHD treatment. Finally, we will perform the first imaging studies of NET in cocaine abusers to determine NET abnormality and availability in abusers as compared to normal controls (Specific Aim 5). We will test the hypotheses that MP at therapeutic doses occupies a significant fraction of NET, and that NET is upregulated in cocaine abusers in agreement with studies in post-mortem brain. _This study will provide the first suitable NET radioligand to investigate, for the first time, the role of NET in the reinforcing as well as the therapeutic properties of stimulants in humans with PET. Moreover, extensive PET investigations in our laboratory of the psychostimulant effects on the dopamine system support the value of this approach to advance our understanding of the processes responsible for addictive, toxic and therapeutic effects of stimulants.